Differential capacitance sensor devices are generally well-known. Modern accelerometers, pressure transducers, and similar transducers often employ a differential capacitance system to detect the null position or motion of a sensing element, e.g., either a proof-mass or diaphragm, as the basic mechanical-to-electrical conversion principle. This is particularly true of Micro Electro-Mechanical System (MEMS) sensor devices in which utilization of differential capacitance is nearly ubiquitous. A major cost driver in the production of sensor devices that use differential capacitance to detect null or motion of the sensing element is a need to trim the capacitive sensing system to null at zero displacement.
Existing differential capacitance detector drive circuits typically apply opposite polarity or “mirrored” excitation signals to a differential sensor capacitor pair. All things being equal, this produces a null signal at a common junction between the capacitor pair as input to a transimpedence amplifier. In a practical sensor, the mirrored excitation signals result in a biased null signal at the common junction at zero displacement of the capacitor pair because the circuit cannot be perfectly balanced and the two sense capacitors forming the capacitor pair cannot be exactly identical. Accordingly, prior art circuits have traditionally introduced front end bias trimming to produce a null signal at zero displacement by addition of a programmable capacitor array.
These additional capacitor arrays may be applied in various configurations. For example, existing differential capacitance detector drive circuits typically utilize either parallel bias trim arrays or drive level bias trim arrays for trimming the capacitive sensing system to null at zero displacement.
FIG. 1 illustrates a typical parallel configuration example of an existing detector drive circuit 1 having a drive portion 2 having a pair of drive signal generators 2a and 2b that are structured to apply opposite polarity or “mirrored” excitation signals to two sense capacitors 3a and 3b of a differential capacitance sensor 3. If the circuit is perfectly balanced and the two sense capacitors 3a, 3b are exactly identical, application of these mirrored excitation signals produces a null at a common junction 4 between the capacitor pair 3a, 3b at zero displacement as input to a transimpedence amplifier 5. In a practical differential capacitance sensor, the signal is biased, or not null, at the common junction 4 at zero displacement of the capacitor pair 3a, 3b. Accordingly, front end bias trimming is introduced via a programmable capacitor array 6 to produce the null signal.
The bias trim capacitor array 6 is switched into the circuit 1 in parallel with a smaller of the two sense capacitors 3a or 3b to bring it into equity with the larger. The null signal now produced at the common junction 4 between the capacitor pair 3a, 3b at zero displacement is input to the transimpedence amplifier 5. Output of the transimpedence amplifier 5 serves as input to a variable gain amplifier 7, which amplified signal is output to a synchronous demodulator 8 whose output is filtered through a filter buffer 9.
FIG. 2 illustrates another common configuration of the detector drive circuit 1 that switches the bias trim capacitor array 6 into the path of the excitation drive signal output by the drive portion 2 to preferentially reduce the applied excitation voltage and thus, the effective signal from that side of the capacitor pair 3a, 3b. 
Such front end bias trimming via the programmable bias trim capacitor array 6, as taught by the prior art, is both costly and difficult to implement adequately. As illustrated, the typical approaches require complex circuitry that is difficult to implement in discrete form, generally requiring a large investment in an application specific integrated circuit.
Therefore, devices and methods for overcoming these and other limitations of typical state of the art balancing of differential capacitance detector drive circuits in MEMS accelerometers, pressure transducers, and other transducer devices are desirable.